1. Field of the Invention
The present invention relates to the use of ionic liquids in the carbonylation of alkyl aromatic compounds to form the corresponding aromatic aldehyde.
2. Description of the Related Art
Carbonylation of an aromatic compound can be carried out by a reaction generally referred to as the Gatterman-Koch reaction. Published in 1897, Gatterman and Koch described the direct carbonylation of various aromatic compounds by the use of carbon monoxide and hydrogen chloride in the presence of aluminum chloride and cuprous chloride (Gatterman, L. and Koch, J. A., Chem. Ber., 30, 1622 (1897)). The reaction was subsequently expanded to include other Lewis acids. Further, it was discovered that the cuprous chloride could be eliminated if the CO pressure was increased. A review of such reactions is set forth in Olah, G. A., "Friedel-Crafts and Related Reactions", Wiley-Interscience, N.Y., Vol. III, 1153 (1964).
U.S. Pat. No. 2,485,237, for example, describes replacing the hydrogen chloride and aluminum chloride catalyst combination with hydrogen fluoride and boron trifluoride. Further use of the HF-BF.sub.3 catalyst is described in U.S. Pat. No. 3,284,508 where the recovery of the fluorides is stated to be improved.
Other catalysts that have been reported for use in a Gatterman-Koch type carbonylation reaction include combinations of Lewis and strong Bronsted acids such as SbF.sub.5 -HF as is described in U.S. Pat. No. 4,218,403. The use of Bronsted superacids alone, such as fluorosulfonic acid or trifluoromethane sulfonic acid, were also reported to be effective catalysts. See for example Olah, G. A., Laali, K., and Farooq, O., J. Org. Chem., 50, 1483 (1985).
These processes generally form an aldehyde-acid catalyst complex that must be dissociated in order to separate the aldehyde product. While the complex can be dissociated by known techniques such as the addition of water to the solution, these techniques may destroy or chemically alter the catalyst thereby making the reuse of the catalyst impossible, impractical, or expensive.
U.S. Pat. No. 4,554,383 recites the use of a "melt" catalyst of aluminum halide and alkyl pyridinium chloride in the selective carbonylation of toluene to tolualdehyde. The yields reported are generally 10% or less. Although not explicitly stated, it appears that the "melt" is what would today be called an ionic liquid. An ionic liquid is a liquid that is composed entirely of ions. Descriptions of ionic liquids can be found in Seddon, K. R., Molten Salt Forum, 5-6, pp. 53-62 (1998) and Seddon, K. R., Kinetics and Catalysts, 37, 5, pp. 743-748 (1996). In this patent, the ionic liquid is comprised of pyridinium.sup.+ and AlCl.sub.4.sup.- ions. Because equimolar amounts of aluminum halide and alkyl pyridinium chloride are combined in making this ionic liquid, the ionic liquid used in the examples is neutral; i.e., neither basic nor acidic.
It would be desirable to provide a process that obtains good conversion of alkyl aromatic compounds to the corresponding alkyl aromatic aldehyde. Further, it would be desirable to provide a process that allows for convenient separation of the aldehyde product from the reaction mixture.